§ 16. The geologist plunges into vague suppositions and fantastic theories in order to account for these cliffs; but, after all that can be dreamed or discovered, they remain in great part inexplicable. If they were interiorly shattered, it would be easy to understand that, in their hardened condition, they had been broken violently asunder; but it is not easy to conceive a firm cliff of limestone broken through a thickness of 2000 feet without showing a crack in any other part of it. If they were divided in a soft state, like that of paste, it is still less easy to understand how any such soft material could maintain itself, till it dried, in the form of a cliff so enormous and so ponderous: it must have flowed down from the top, or squeezed itself out in bulging protuberance at the base. But it has done neither; and we are left to choose between the suppositions that the mountain was created in a form approximating to that which it now wears, or that the shock which produced it was so violent and irresistible, as to do its work neatly in an instant, and cause no flaws to the rock except in the actual line of fracture. The force must have been analogous either to the light and sharp blow of the hammer with which one breaks a stone into two pieces as it lies in the hand, or the parting caused by settlement under great weight, like the cracks through the brickwork of a modern ill-built house. And yet the very beds which seem at the time they were broken to have possessed this firmness of consistency, are also bent throughout their whole body into waves, apparently following the action of the force that fractured them, like waves of sea under the wind. Truly the cloud lies darkly upon us here!
| Fig. 13. |
§ 17. And it renders these precipices more remarkable that there is in them no principle of compensation against destructive influences. They are not cloven back continually into new cliffs, as our chalk shores are by the sea; otherwise, one might attribute their first existence to the force of streams. But, on the contrary, the action of years upon them is now always one of deterioration. The increasing heap of fallen fragments conceals more and more of their base, and the wearing of the rain lowers the height and softens the sternness of their brows, so that a great part of their terror has evidently been subdued by time; and the farther we endeavor to penetrate their history, the more mysterious are the forms we are required to explain.
§ 18. Hitherto, however, for the sake of clearness, we have spoken of hills as if they were composed of a single mass or The three great representative forms of stratified mountains. volume of rock. It is very seldom that they are so. Two or three layers are usually raised at once, with certain general results on mountain form, which it is next necessary to examine.
1st. Suppose a series of beds raised in the condition a, [Fig. 13], the lowest soft, the uppermost compact; it is evident that the lower beds would rapidly crumble away, and the compact 1. Wall above slope. mass above break for want of support, until the rocks beneath had reached a slope at which they could securely sustain themselves, as well as the weight of wall above, thus bringing the hill into the outline b.
| Fig. 14. |
2d. If, on the other hand, the hill were originally raised as at c, the softest beds being at the top, these would crumble into their smooth slope without affecting the outline of the mass below, 2. Slope above wall. and the hill would assume the form d, large masses of débris being in either of these two cases accumulated at the foot of the slope, or of the cliff. These first ruins might, by subsequent changes, be variously engulfed, carried away, or covered over, so as to leave nothing visible, or at least nothing notable, but the great cliff with its slope above or below it. Without insisting on the evidences or probabilities of such construction, it is sufficient to state that mountains of the two types, b and d, are exceedingly common in all parts of the world; and though of course confused with others, and themselves always more or less imperfectly developed, yet they are, on the whole, singularly definite as classes of hills, examples of which can hardly but remain clearly impressed on the mind of every traveller. Of the first, b, Salisbury Crags, near Edinburgh, is a nearly perfect instance, though on a diminutive scale. The cliffs of Lauterbrunnen, in the Oberland, are almost without exception formed on the type d.
3d. When the elevated mass, instead of consisting merely of two great divisions, includes alternately hard and soft beds, as at a, [Fig. 14], the vertical cliffs and inclined banks alternate with 3. Slope and wall alternately. each other, and the mountain rises on a series of steps, with receding slopes of turf or débris on the ledge of each, as at b. At the head of the valley of Sixt, in Savoy, huge masses of mountain connected with the Buet are thus constructed: their slopes are quite smooth, and composed of good pasture land, and the cliffs in many places literally vertical. In the summer the peasants make hay on the inclined pastures; and the hay is "carried" by merely binding the haycocks tight and rolling them down the slope and over the cliff, when I have heard them fall to the bank below, a height of from five to eight hundred feet, with a sound like the distant report of a heavy piece of artillery.
| Fig. 15. |